Method for discriminating between objects

ABSTRACT

A method for discrimination between a first ( 308 ) and a second surface ( 304 ) type based on reflectivity has a light source ( 416 ) that illuminates on a media surface. A photosensor ( 420 ) receives and measures the reflection value from the surface. A first gain element adjusts a voltage from the photosensor and a second gain element adjusts a current measurement supplied to the light source. A subtractor ( 530 ) for subtracting the first adjusted voltage ( 534 ) and the adjusted measurement ( 538 ) are subtracted to provide an output value close to zero with respect to the second surface and near a maximum with respect to the first surface. The adjusted reflection value and a threshold reference value ( 428 ) are compared ( 124 ) and indicates whether the first surface or the second surface is present.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned copending U.S. patent applicationSer. No. ______ (Attorney Docket No. 96545/NAB), filed herewith,entitled METHOD FOR DISCRIMINATING BETWEEN OBJECTS, by Burkatovsky; thedisclosure of which is incorporated herein.

FIELD OF THE INVENTION

The present invention relates in general to a method for discriminatingbetween two objects based on surface reflectivity differences, and morespecifically to discriminating between printing plates covered by apolymer emulsion and interleaf paper between the plates.

BACKGROUND OF THE INVENTION

A computer-to-plate (CTP) device 204, shown in FIG. 2, is used fordirect imaging on printing plates. The plates are loaded in a magazineor cassette and delivered one by one to be exposed by the imagingdevice. Alternatively the plates can be provided by an automatic plateloader (APL) 104, wherein a plates stack 108 is inserted into the APL asis shown in FIG. 1. The plates 304 provided in the cassette or in aplate stack 104 are usually separated by interleaf paper 308, shown inFIG. 3, interposed between the plates to prevent the emulsion-coveredsurfaces of the plates from being damaged.

In the course of imaging plates, the plate placed at the top of thestack is picked and transferred to the exposure area for imaging. Whenan interleaf paper (slip-sheet) is at the top of the stack, the paper ispicked and disposed of, before picking the plate. There is a need fordiscriminating between plate and the interleaf paper are used, tocorrectly identify the topmost object on the stack.

U.S. Pat. No. 6,825,484 (Burkatovsky) describes a discriminating devicebased on measurements of the light reflections from the surfaces withdifferent roughness. For example, discrimination between paper andnon-covered by emulsion printing plate will be reliable due tosubstantially different roughness of paper as opposed to a smooth andglossy plate metal surface. But discrimination between paper andemulsion covered plate will often be inaccurate due to the smalldifference between their roughness properties.

Another method for discriminating the slip sheets and emulsion coveredprinting plate described in U.S. Pat. No. 7,157,725 (Kawamura). Thismethod is based on the difference between absorbance (reflectance) of aslip sheet and a plate, irradiated by light of 570-740 nm wavelengths.

Reflectance of emulsions and papers produced by different manufacturersmay vary substantially. An example of reflections from papers andemulsions of different manufacturers is shown in FIG. 6. X-axis depictslighting source current or irradiating intensity I and Y-axis depictsthe reflection intensity represented by photosensor output voltage Vr.The reflections from paper slip sheet and emulsion covered platemanufactured by the first manufacturer and measured at predeterminedheight and light source current is shown by lines 604 and 608respectively. Reflections from paper slip sheet and emulsionmanufactured by the second manufacturer and measured at the same heightand light source current is shown by lines 612 and 616. The thresholdVth1 for discrimination between paper and emulsion produced by the firstmanufacturer according to Kawamura et al., should have value greaterthan emulsion reflection and less than paper reflection values. Whileirradiation caused by the same light source current the reflections frompaper and emulsion produced by the second manufacturer are smaller. Thismight happen due to different processes applied for emulsion coveredprinting plates and slip sheet by different manufacturers. In this casethe chosen threshold Vth1 will be greater than paper and emulsionreflections and media discriminating will be impossible.

It should be noted that not only manufacturer media variations anddifferences between the batches of media lead to reflection deviations.Changing parameters such as distance to media, light source, ambientlight are also impact on reflections thus making difficult to practicalimplementation of the method suggested by Kawamura et al.

The purpose of this invention is to improve the paper slip sheet andemulsion covered plate discrimination capability.

SUMMARY OF THE INVENTION

Briefly, according to one aspect of the present invention a method fordiscrimination between a first and a second surface type based onreflectivity has a light source that illuminates on a media surface. Aphotosensor receives and measures the reflection value from the surface.A first gain element adjusts a voltage from the photosensor and a secondgain element adjusts a current measurement supplied to the light source.A subtractor for subtracting the first adjusted voltage and the adjustedmeasurement are subtracted to provide an output value close to zero withrespect to the second surface and near a maximum with respect to thefirst surface. The adjusted reflection value and a threshold referencevalue are compared and indicates whether the first surface or the secondsurface is present.

The invention and its objects and advantages will become more apparentin the detailed description of the preferred embodiment presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an automatic plate loader (APL)loaded with stack of plates (prior art);

FIG. 2 shows an imaging device connected to an APL (prior art);

FIG. 3 is a schematic illustration of a plates stack, showing platesseparated by slip sheets (prior art);

FIG. 4 is a schematic illustration of paper/plate discrimination deviceknown in the art;

FIG. 5 is a schematic illustration of the discrimination device proposedby the current disclosure;

FIG. 6 is shows a behavior function of reflection from various mediaobjects;

FIG. 7 is shows a graph of amplification factors selection; and

FIG. 8 is shows the discrimination behavior of the suggested device inresponse to the distance from the measured target.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the disclosure.However, it will be understood by those skilled in the art that theteachings of the present disclosure may be practiced without thesespecific details. In other instances, well-known methods, procedures,components and circuits have not been described in detail so as not toobscure the teachings of the present disclosure.

While the present invention is described in connection with one of theembodiments, it will be understood that it is not intended to limit theinvention to this embodiment. On the contrary, it is intended to coverall alternatives, modifications, and equivalents as covered by theappended claims.

The schematic illustration of a discriminating device known the art isshown in FIG. 4. FIG. 5 shows components of the proposed invention,where some of the components are also being used in the Kawamura et al.,as are shown in FIG. 4. The illumination control unit 404 intended forsetting the level of media 408 illumination connected through the lightsource driver 412 to the light source 416, the light source can be a LED(light emitting diode). The reflections from media 408 are measured byphotosensor 420.

An adjustment element 542 is connected by its inputs to the outputs ofthe photosensor 420 and the illumination control unit 404. The output ofthe adjustment element 542 is connected to the first input of comparator124 while the second input of comparator 124 is connected to thethreshold reference 428.

The irradiation of the tested media provided by the light source 416controlled by the illumination set point signal Vi produced byillumination control unit 404 through the light source driver 412. Thephotosensor 420 generates signal V_(R) proportional to the reflectionfrom tested media. This signal is amplified by first input amplificationfactor 534 (G_(R)) of adjustment element 542, simultaneously theillumination set point signal Vi is gained (divided) by second inputamplification factor 538 (Gi) and than subtracted by subtractor 530 fromthe gained V_(R) signal. The result of the subtraction is represented bysubtractor output value signal V_(S) complying with following equation:

V _(S) =V _(R) *G _(R) −V _(i) *G _(i)  (1)

FIG. 7 explains the selection of G_(R) and Gi which is substantial forsuggested discriminating device. Assuming G_(R0)=1 and Gi₀=0.

In this case according to Equation (1) VS will equal to VR.

V _(S) =V _(R)  (2)

In the case when VS equals VR, according to Equation (1), the behaviorof the proposed discrimination device shown in FIG. 5 will behave as thedevice described by Kawamura et al, which is shown in FIG. 4.

Line 704 in FIG. 7 shows a response function representing the reflectionfrom paper slip sheet and line 708 represents reflection from emulsioncovered plate surface respectively. These lines are identical to lines604 and 608 of FIG. 6. Respectively Vsp=Vpl and Vse=Vel. The relationbetween the Vsp and Vse determines the capability of the device toperform a reliable discrimination between paper slip sheets and emulsioncovered plates. The relation between amplified paper reflection andamplified (divided) emulsion reflection is defined as discriminationfactor DF.

DF=Vsp/Vse  (3)

As much as the Vsp value is bigger than Vse value, the discriminationwill be more reliable, due to covering of a larger reflection range andthus decreasing the sensitivity of reflection deviations.

In other words in order to improve the discrimination capability of adiscrimination device the value of DF needs to be increased. This can beachieved by adjusting the amplification factors 534 (G_(R)) and 538(Gi). The Gi adjustment should be provided while emulsion coveredprinting plate is examined. Adjustment may start with mentioned abovevalues of Gi₀ and G_(R0). (Gi_(O)=0, G_(RO)=1). According to equations(2) and (3) discrimination factor for these values will be defined as

DF₀ =Vsp ₀ /Vse ₀.  (4)

Now by increasing 538 (Gi) up to the moment when Vse will be close tozero we obtain the situation when Vse is practically not dependent uponthe light source 416 current and remains low within the light sourcecurrent possible range (line 716). Respectively after adjusting 538 (Gi)while examining the paper slip sheet, the Vsp line 704 will change itsslope. The Vsp dependence on light source 416 current after Giadjustment is presented by line 712.

Increasing the light source 416 current to Im by means of illuminationcontrol unit 404 we obtain Vsph value while examining paper and Vselvalue while examining emulsion covered plate. As Vsph is bigger than Vspand Vsel is lower than Vse thus according to equation (3) the value ofrepresenting discrimination factor

DF1=Vsph/Vsel  (5)

will be much bigger than DF₀ (4), thus yielding a substantially improveddiscrimination capabilities. The maximum value of Vsph is restricted bypower supply voltage. In other words the threshold margin is enough tosupport the discrimination of plates and emulsions from variousmanufacturers.

It should be noted that the DF1 value may be achieved also without lightsource 416 current changing (from In to Im). This can be obtained byincreasing the amplification factor 534 (G_(R)) while maintaining line716 close to the X-Axis (as is shown in FIG. 7), by adjusting theamplification factor 538 (Gi).

FIG. 8 depicts the subtractor output value V_(S) signal used in FIG. 7as a function of media to sensor distance shown as axis H (804).Comparing to FIG. 7 point Hn of axis H determines the media to sensordistance while illuminating current value is In. Reflections from paperslip sheet and emulsion covered plate at this point are the same asshown on FIG. 6 (Vpl and Vel). Respectively the subtractor output valueswhile Gi_(O)=0 and G_(RO)=1 are Vsp and Vse, the same as shown on FIG.7. Now while maintaining the constant value of LED current. In theamplification factors 534 (G_(RO)) and 538 (Gi_(O)) should be adjustedsuch as subtractor output value V_(S) is close to zero Vsel in responseto lower reflectance surface type (emulsion covered plate). As a resultof the subtractor output value V_(S) is maximal Vsph in response tohigher reflectance surface type (slip slit paper). The subtractor outputvalue behavior represents the emulsion covered plate reflection aftergains adjustment is depicted by line 812. Respectively the subtractoroutput value behavior for the slip sheet paper reflection after gainsadjustment is represented by behavior function 808 (sensor to mediadistance is bigger than Hn) and line Vsph—maximum voltage valuerestricted by power supply voltage (sensor to media distance is smallerthan Hn).

As it can be seen from FIG. 8 the range from smaller media to sensordistance Hm to higher media to sensor distance Hn shows practically samesubtractor output values such as Vsel close to zero in the case ofemulsion testing and Vsph close to power supply voltage in case of slipsheet paper testing. Within this (Hm−Hn) range the discrimination factorhas maximum allowable value DF=(Vsph/Vsel) according to Equation (3) andDF=constant as well. Referring to the prior art performance according toFIG. 8, in the Hn sensor to media distance point the prior art devicewill have a worse DF than the suggested device as is shown by Equations(4) and (5). In addition the Hm sensor to media distance point in priorart device will not work at all, due to very high incoming reflection.Specific gains adjustment allows discrimination performance practicallyindependent of sensor to media distance and extending of sensor to mediadistance range where discriminating is possible.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention.

PARTS LIST

-   104 automatic plate loader-   108 plate stack-   124 comparator-   204 computer-to-plate (CTP) device-   304 plates (with emulsion surface up)-   308 interleaf paper (slip sheets)-   404 illumination control unit-   408 media-   412 light source driver-   416 light source-   420 photosensor-   428 threshold reference-   530 subtractor-   534 first input amplification factor-   538 second input amplification factor-   542 adjustment element-   604 sheet slip reflection graph a first plate manufacturer-   608 emulsion surface reflection graph a first plate manufacturer-   612 sheet slip reflection graph from a second plate manufacturer-   616 emulsion surface reflection graph a second plate manufacturer-   704 slip sheet reflection graph a first plate manufacturer-   708 emulsion surface reflection graph a first plate manufacturer-   712 adjusted slip sheet reflection graph-   716 adjusted emulsion surface reflection graph-   804 height axis-   808 slip sheet reflection behavior function after gains-   812 emulsion surface reflection behavior function after gains

1. A method for discriminating between a first surface type and a secondsurface type wherein said first surface reflectivity is greater thansaid second surface reflectivity comprising: illuminating said first orsaid second surface type with a light source; measuring a reflectivityof said first surface type or said second surface type; adjusting a gainof a voltage or current based on said measured reflectivity; adjusting ameasurement of a current or a voltage supplied to said light source;subtracting the first adjusted voltage or current from the adjustedmeasurement to provide a subtractor output value; providing a thresholdreference value; and comparing said subtractor output value with saidthreshold reference value to determine whether said first surface typeor said second surface type is present.
 2. The method according to claim1 wherein said light source is a LED.
 3. The method according to claim 1wherein said first surface type is an emulsion covered surface.
 4. Themethod according to claim 1 wherein said second surface type is a slipsheet paper.